This project will analyze various methods for performing antimicrobial susceptibility tests of Mycobacterium avium complex (MAC), with the goal of determining which test method correlates best with clinical response in the patient. Such a study is needed because little information exists relating in the in vitro activity of drugs with clinical response to drug treatment, and because there are a number of different methods for performing drug susceptibility tests against MAC, each giving different data and each with its own advantages and disadvantages. The methods that will be compared in this study include measurements of the activity of drugs i) in broth medium, ii) inside a mouse macrophage-derived cell line (J774), and iii) inside alveolar macrophages obtained from HIV-infected patients. The MAC strains to be used in these experiments are strains isolated from patients enrolled in ACTG protocol 135, a clinical trial comparing a 4-drug versus a 5-drug regimen for treatment of disseminated MAC infection. Susceptibility tests will be performed using common broth culture techniques to measure survival of MAC after exposure to each of the single drugs and to the 4-drug and 5-drug combinations used in treatment of patients in ACTG 135. In addition, mouse J774 cells and alveolar macrophages from HIV-infected patients will be infected with these MAC strains then treated with the same antimicrobial agents, using both single drugs and drug combinations. Survival of MAC inside drug-tested cells and control (untreated) cells will be followed over time. A major objective of this study will be to determine under what conditions the simpler broth and J-cell methods can be used to reliably assess the in vitro activity of drugs against MAC. This will be accomplished by comparing the quantitative data obtained from the three in vitro methods will clinical data obtained in ACTG 135. The results of these experiments will provide the most complete set of data to date on the relationship between in vitro MAC susceptibility test results and clinical response to treatment. The results will also aid in the development of standardized methods for performing drug susceptibility against MAC.